My grade 9’s are doing static electricity at the moment, and I am experiencing some weirdness and frustration with the kits I have. Firstly I have a combination of new and old kits, but they all come with a standard collection of items – glass, ebonite and plexiglass rods, fur, felt and silk pads, and a few other odds and ends. My problem is I am finding it hard to reliably, and consistently, get a good charge, especially a positive charge.
The instructions usually suggest rubbing glass with silk to produce a positive charge on the glass. This, in my experience, never works. Last year I discovered that rubbing the glass rod with the plastic baggie the kit comes in worked well – but with the new kits I just received that doesn’t seem to work, no idea why not.
And then there is the mysterious flip-flopping plexiglass. Acrylic when rubbed with fur should become negative – but when I held it up to a negatively charged electroscope, the leaves dropped – indicating a positive charge. I tested it with a positively charged electroscope, and the leaves spread apart, confirming the positive charge. Okay, so it’s positive. But then when I tried to demo that it was positive, it suddenly had the opposite effect. It was bizarre, and my students were totally confused – as was I.
I love a teachable moment as much as the next guy, but when the message seems to be that science is arbitrary and unpredictable, I don’t think it is the right lesson to convey. I hate it when that happens.